NEW METOPOSAURS FROM THE SOUTHWESTERN UNITED STATES AND THEIR PHYLOGENETIC RELATIONSHIPS by Beth Davidow-Henry, B.S. A THESIS IN MUSEUM SCIENCE Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Requirements for the Degree of MASTER OF ARTS Approved U- Chairperson of the Committee dL L,4 Jr ' AccepteAr'r<»ntpdH yO^^^^t^ll^ the Graduate School December, 1987 /f(L r^. (*^QO. ^ ACKNOWLEDGEMENTS Special thanks are due to Drs. Donald Baird (Princeton University) , John Bolt (Field Museiam) , Joseph Gregory (University of California, Berkeley), John Mecham (Texas Tech University), and Michael Morales (Museum of Northern Arizona) for their many helpful discussions and suggestions. I am especially indebted to Dr. Kent Rylander (Texas Tech University) who patiently guided me through the course of this thesis. And to Dr. Michael Willig of Texas Tech University, a special thank-you for taking time at the last minute to run the computer programs for me. Many thanks go to Dr. Sankar Chatterjee, Chairperson of my committee and major professor, for his support, interest, and assistance with this project. I thank my husband, Greg, not only for his assistance with the photography, but also for being there when it was important. And, for their steady support and never-ending faith in me, I chank my fa'mily, especially Mom and Dad. 11 TABLE OF CONTENTS ACKNOWLEDGEMENTS ii LIST OF FIGURES iv LIST OF TABLES v LIST OF PLATES vi 1. INTRODUCTION 1 2. MATERIALS AND METHODS 9 3. GEOLOGIC SETTING 11 4. DESCRIPTION OF NEW METOPOSAURS 16 Metoposaurs without otic notches 16 Metoposaurs with otic notches 30 5. RESULTS 34 Cartesian transformations 37 Biometric analyses 42 6. DISCUSSION 63 Growth studies and statistical analyses 63 Characters for taxonomic evaluation 63 7. CONCLUSIONS • • 70 REFERENCES 73 111 LIST OF FIGURES 1. Skeletal reconstruction of a metoposaur (after Romer, 1966) 3 2. Illustrations of the three genera of metopoaauroids showing the otic notch and tabular processes of: (A) Dictyocephalus; (B) Anaschisma; and (C) Metoposaurus 6 3. Dockum Formation in West Texas and type section of the Cooper Creek Member at the Post Quarry in Garza County, Texas (after Chatterjee, 1986) 13 4. Key to skull measurements and abbreviations 36 5. Cartesian transformation series of metopoaauroids: (A) TTUP 9216; (B) TMM 31099-12B; (C) TTUP 9083 (D) TMM 31100-124; (E) TMM 31100-42; (F) TTUP 9083 40 6. Biometric analyses: Top figure-logarithmic graph; (Figures 6A-6P) Bottom figure-ANOVA results 48 7. Two groups of metoposauroids 66 8. Cladogram showing the relationships of the genera of metoposaurs with the capitosaurs chosen for outgroup comparison 69 IV LIST OF TABLES 1. Table of skull measurements 38 2. Table of measurements as entered into the StatWorks computer program including natural logarithmic values for each variable 44 LIST OF PLATES 1. Dorsal view of TTUP 9216 18- 2. Ventral view of TTUP 9216 22 3. Dorsal view of TTUP 9237 26 4. Dorsal view of UCMP 82/39/37 29 5. AMNH 5661; Dictyocephalus elegans Leidy, 1856 32 6. New specimens in the genus Dictyocephalus (left to right) TTUP 9216; TTUP 9237; UCMP 82/39/37 72 VI CHAPTER 1 INTRODUCTION Metoposaurs are characteristic Late Triassic labyrinthodont amphibians recorded from North America, Germany, Morroco, and India. The family Metoposauridae, named from the Europena genus Metoposaurus, is distinguished by its flat skull and the position of the orbits anterior to the midlength of the skull table. Bones of the skull roof are deeply sculptured on their dorsal surfaces. Skeletons of the predominantly aquatic metoposaurs indicate an animal often 2 meters in length (see Fig. 1). Metoposaurs have been known from the Late Triassic Dockum sediments of West Texas since Case described a specimen in 1920 upon which he erected the genus Butteneria. Since then, five species have been named from the Dockum, all belonging to this genus. There is considerable disagreement among paleontologists concerning metoposaur taxonomy, including those specimens from the Dockum. Colbert and Imbrie (1956) reviewed the Family Metoposauridae and concluded that the American specimens were sufficiently distinct from the Eurasian specimens to warrant placing the former into the genus Eupelor Cope, 1868, as distinct from the genus Metoposaurus Lydekker, 1890, in which they placed all the Eurasian specimens. Thus, Butteneria was reclassified as Eupelor according tu their vork. Figure 1. Skeletal reconstruction of a metoposaur (after Romer, 1966). ,-y} : b- /"\ ^\-: •••-A /•• / \ • \ . ^ / A\ ;^ < ••..-^:T'- 5^'••:u./.rNi xV, J;\ More recently, Roy-Chowdhury (1965) doubted that the differences between the Eurasian and American specimens were great enough to justify placing them in two separate genera. He proposed placing all species in the genus Metoposaurus, which takes precedence over all other generic names. According to his taxonomy, which is followed in this study, all of the Dockum forms are congeneric. Morover, Roy-Chowdhury merged all of the Dockum Formation specimens into a single species, Metoposaurus fraasi. Metoposaurus fraasi (see Fig. 2) is characterised by a robust skull with deeply incised otic notches. Tabular processes, or horns, are also present. The pineal foramen of M. fraasi is situated on the rear third of the parietal elements. There is much interspecific variation in this group and many features have been altered by post-mortem crushing, two reasons why its members have been assigned to so many species. Two other genera of metoposaurs are recognised. The first of these is represented by a solitary specimen, Dictyocephalus elegans described by Joseph Leidy in 1856, which survives today as a fragmentary specimen of a small metoposauroid. ^. elegans is characterised not only by its small size but also by the absence of well-defined otic notches and tabular horns. An interesting feature of Dictyocephalus is the location of the pineal foramen centered, rather than near the rear, on the parietals. Colbert and Imbrie suggested that Dictyocephalus may represent a juvenile stage of a large metoposaur. Contrary to their suggestion. Dr. Donald Baird (pers. comm.) believes that Dictyocephalus is a valid genus based upon the aforementioned characters. Figure 2. Illustrations of the three genera of metoposauroids showing the otic notches and tabular processes of: (A) Dictyocephalus; (B) Anaschisma; and (C) Metoposaurus. CJ Dr. Joseph Gregory revised the taxonomy of the metoposaurs on the basis of the presence or absence of the otic notch and tabular horns. He concluded that for those specimens lacking tabular horns and bearing a shallow otic notch the genus Anaschisma Branson, 1905, was vaild. Anaschisma is founded upon two specimens which come from the Popo Agie Member of the Chugwater Formation of Wyoming. These specimens have shallow otic notches and lack tabular horns. Dr. Baird (pers. comm.) suggests that Dictyocephalus may represent a juvenile individual of Anaschisma based upon these shared characters as seen in figure 2. If that proves to be the case, then Anaschisma would be the junior synonym of Dictyocephalus. However, the systematic position of Anaschisma is questionable because of the incompleteness of the material. Two specimens, one 45 cm the other 41 cm in length,were recovered from near each other from the Carnian-aged sediments of the Popo Agie. One other specimen, currently undescribed, is known from the Redonda Formation of northeastern New Mexico. That specimen is approximately 20 cm in length. In 1984, a skull of a small metoposauroid was discovered from the Dockum Formation near Post, Texas. This skull, TTUP 9216, is of particular interest in that it is the smallest individual known with a length of 6.8 cm. The skull features are also unusual when compared with the other metoposauroids from the area. The study of TTUP 9216, the specimen which forms the basis for this thesis, is supplemented by two other new metoposauroid specimens: UCMP 82/39/37 from Petrified Forest National Park, and TTUP 9237 from Crosby County, Texas. 8 The purpose of this study is twofold: (a) to describe the morphology of the newly discovered skulls; and (b) to compare them to skulls from other specimens in the three existing genera as a means of evaluating their taxonomic position within the Family Metoposauridae. CHAPTER 2 MATERIALS A1>ID METHODS The newly discovered skull, designated as TTUP 9216, was recovered from the Post Quarry, Garza County, Texas. Discovered by Michael W. Nickell during excavation, the small skull was encased in a matrix of red clay. To remove the skull from the matrix was a slow process requiring much care due to its delicate nature. Acetone was used to soften the matrix in order to avoid moisture damage to the bones. Fine dental probes were used to pick away the damp matrix which was then gently brushed off. A calcareous crust covering the anterior portion of the skull near the external nares was removed with a weak solution of hydrochloric acid. Specimen repositories are identified by the abbreviations preceding the specimen number: TMM, Texas Memorial Museum; TTUP, The Museum, Texas Tech University; UCMP, University of California Museum of Paleontology, Berkeley; YPM, Yale Peabody Museum; AMNH, American Museum of Natural History. Nine specimens were used in this study: 1. TTM 31100-42: well-preserved complete skull, portion of premaxilla anterior of external nares missing; Howard County, Texas; skull length = 455mm. 10 2. TMM 31100-124: well-preserved skull; Howard County, Texas; skull length = 400mm. 3. TMM 31099-12B: excellent skull of juvenile metoposaur complete with both stapes and lower mandibles; Howard County, Texas; skull length = 160mm. 4. TTUP 9042: well-preserved skull; Howard County, Texas; skull length = 380mm. 5. TTUP 9083: large complete skull, right side partially distorted by crushing; Crosby County, Texas; skull length = 630mm. 6. AMNH 5661: type specimen of Dictyocephalus elegans, partially intact posterior region; Cumnock Formation, North Carolina; skull length estimated at 78mm.